1. Field of the Invention
The present invention relates to a video cassette recorder (VCR), and more particularly to an apparatus for separating vertical synchronizing signals from image signals in the VCR.
2. Description of the Prior Art
For promoting the understanding of the present invention, scanning and synchronization will be described, before describing the prior art.
As shown in FIG. 1, image signals played back from a VCR tape are displayed on a cathode ray tube (CRT) of a monitor or a television in a sequential manner that a first signal line is displayed on the uppermost portion of the CRT in the right direction and a return is %hen made in the left direction for displaying a next signal line, that is, a second line 2 in the same right direction, as shown in FIG. 1.
Such a sequential displaying of image signal lines is called a scanning.
For displaying a frame as shown in FIG. 1, substantial time is taken, in that 525 scanning times for one frame are required. This 525 scanning times corresponds to the broadcast system of National Television System Committee (NTSC) and means the number of scanning lines constituting one frame.
Also, the substantial scanning time means that a flickering occurs on a screen. In actuality, most television systems adopt an interlaced scanning in which scanning is made for every other scanning lines and then for remaining scanning lines, so as to avoid the flickering phenomenon. In other words, 262.5 scanning times (1/2 of 525 scanning times) repeated two times for displaying one frame.
Scanning carried out in the horizontal direction is called a horizontal scanning, whereas scanning carried out in a vertical direction is called a vertical scanning.
In an interlaced scanning of the NTSC broadcast system, 30 frames are displayed in a second. Each frame is also displayed by two interlaced scannings. That is, the time taken for the scanning from a point a to a point c via a point b is 1/60 second, in FIG. 2.
In FIG. 2, the point a is a start point for making a first frame, the point b an end point for making the first frame, the point c a start point for making a second frame, and the point d an end point for making the second frame.
For displaying one image signal line, accordingly, a time of 1/15,750 second is taken, as expressed in the following equation (1): EQU 1/30 (second).div.525=1/15,750 (second) (1)
For displaying accurately one frame, the same point should be scanned at the transmitting side and the receiving side at the same time. Such a scanning is called a synchronization. Herein, the transmitting side means a VCR tape, whereas the receiving side means a CRT of a monitor or a television.
Accordingly, each image signal recorded on the VCR tape includes both horizontal synchronizing signal components and vertical synchronizing signal components. The horizontal synchronizing signal components are signals for achieving the sequential return carried out in a direction from the rightmost end of the screen and to the leftmost end of the screen. On the other hand, the vertical synchronizing signal components are signals for achieving the sequential return carried out in a direction from the lowermost end of the screen and to the uppermost end of the screen.
When horizontal synchronizing signal components are omitted, inclined lines are generated on a frame. On the other hand, when vertical synchronizing signals are omitted, the frame is moved downwardly or upwardly.
FIG. 3 is a waveform of NTSC broadcast signals in the interlaced scanning system.
By virtue of the above-mentioned reason, it is very important to accurately separate horizontal and vertical signals from an image signal recorded on a VCR tape, upon the play-back operation of VCR.
Referring to FIG. 4, there is illustrated a conventional apparatus for separating vertical synchronizing signal components from image signals. As shown in FIG. 4, the apparatus comprises a head 2 for playing back frequency-modulated image signals recorded on a VCR tape 1, an amplifying unit 3 for amplifying the frequency-modulated image signals played back by the head 2 at a predetermined amplification factor, a demodulating unit 4 for demodulating an output signal from the amplifying unit 3 into an original composite signal comprising a color signal component and a luminance signal component, and a signal detecting unit 5 for detecting vertical synchronizing signal components V.sub.sync from the original composite image signal.
The construction of the signal detecting unit 5 is well known and thus its description will be omitted. The head 2 may be a head having a standard play (SP) function or a long play (LP) function. It may be also a head having a combined SP/LP function. In either case, the head is applied to VCRs of the double rotation head helical scanning system.
Now, the operation of the above-mentioned conventional apparatus will be described.
First, the head 2 plays back image signals recorded on the VCR tape 1 and sends them to the amplifying unit 3. Generally, the image signals recorded on the VCR tape 1 are signals obtained by frequency-modulating original composite image signals.
The amplifying unit 3 amplifies the received image signals at a predetermined amplification factor and then send them to the demodulating unit 4. The demodulating unit 4 demodulates the image signals received from the amplifying unit 3, to obtain original composite image signals.
The signal detecting unit 5 detects vertical synchronizing signal components from composite image signals shown in FIG. 3 and outputs them. These vertical synchronizing signal components V.sub.sync importantly serves to distinguish frames from one another.
However, the above-mentioned apparatus has the following problems. Generally, a VCR currently used has a head having a width of 49 .mu.m for the SP mode or a head having a width of 30 .mu.m for the LP mode. In most VCRs of the double rotation head helical scanning system, a combined SP/LP type head having a width of about 37 .mu.m is used for reducing the manufacture cost, since the head accounts for a considerable portion of the manufacture cost. In all cases of using heads for the SP mode, the LP mode and the combined SP/LP mode, however, a problem occurs upon separating vertical synchronizing signal components from image signals played back during SP/LP or variable playback operations.
For example, where the VCR is equipped with a combined SP/LP type head having a width of about 37 .mu.m and performs the playback mode (QUE mode or REVIEW mode) or the SP/LP mode, the guard band is greatly increased, as shown in FIG. 6, when the combined SP/LP type head carries out a variable playback operation at a velocity of 5 times or a normal SP/LP operation. This is because the width (37 .mu.m) of the head is less than a predetermined width (49 .mu.m) by about 12 .mu.m.
In the VCR having two heads A and B for SP/LP operation, its head A plays back only signals on channels A, without playing back signals on channels corresponding to the head B, as shown in FIG. 6. As a result, the signals on the channels B become guard bands, so that the substantial guard bands are increased even further, as compared with predetermined guard bands. These guard bands causes noise bars during the variable playback operation or the SP/LP operation of VCR, since no image signal is recorded on the guard bands.
On the other hand, a guard band with a small width is provided at the right edge portion of each image track of VCR tape, so as to isolate adjacent image tracks from each other during the SP/LP operation of VCR, as shown in FIG. 7. Where the combined SP/LP type head with the small width of 37 .mu.m is used, the guard bands are also increased.
That is, the amplifying unit 3 of FIG. 4 outputs no frequency-modulated image signal or frequency-modulated image signals with a small level at guard band intervals K, as shown in FIG. 5a. As a result, the demodulating unit 4 of FIG. 4 outputs signals with severe noise bars at the guard band intervals K, as shown in FIG. 5b.
Such noise bars are generally exhibited with a level lower than the low level of vertical synchronizing signal components. Due to such noise bars, accordingly, the signal detecting unit 5 of FIG. 4 which is adapted to detect only the vertical synchronizing signal components with the low level can not detect accurately the vertical synchronizing signal components from the demodulated signals.
As apparent from the above description, the conventional VCRs of the double rotation head helical scanning system can not separate vertical synchronizing signal components from image signals, in all cases of using a single type head for a SP or LP mode and a combined type head for SP/LP mode. As a result, such VCRs can not also performs accurately an on screen display (OSD) function.